1. Field of the Invention
The present invention has for its object devices and processes for the heat insulation of at least one underwater pipe at great depth.
The technical sector of the invention is the domain of the manufacture and assembly of an insulation system outside and around the pipes in which hot effluents, of which it is desired to limit heat losses, circulate.
This invention is applied more particularly to the developments of deep-sea oil fields, i.e. oil rigs installed on the open sea, in which the surface equipment is generally located on floating structures, the well heads being on the sea bed. The pipes concerned by the present invention are either links between well heads, or the part resting on the bed of the bed/surface links.
2. Description of the Related Art
Deep-sea developments are at the present time effected at water depths reaching 1500 meters. Future developments are envisaged at depths of up to 3000 meters and more.
The principal application of the invention is the heat insulation of pipes or lines immersed under water and more particularly at great depth, beyond 300 meters, and conveying hot oil products of which too great a cooling would be problematic both during normal production and in the case of production being stopped.
In effect, in this type of application, numerous problems are raised if the temperature of the oil products decreases by a significant considerable value with respect to their production temperature which is generally beyond 60 to 80xc2x0 C., while the temperature of the surrounding water, especially at great depth, may be less than 10xc2x0 C. If the oil products cool for example below 30xc2x0 to 60xc2x0 C., for an initial temperature of 70 to 80xc2x0 C., the following is generally observed:
a considerable increase in the viscosity which then reduces the flowrate of the pipe,
a precipitation of dissolved paraffin which then increases the viscosity of the product and of which the deposit may reduce the useful internal diameter of the pipe,
the flocculation of the alphaltenes inducing the same problems,
the sudden, compact and massive formation of gas hydrates which precipitate at high pressure and low temperature, thus suddenly obstructing the pipe.
Paraffins and alphaltenes remain attached to the wall and then require cleaning by scraping the inside of the pipe; on the other hand, the hydrates are still more difficult, and even impossible to remove.
The function of the heat insulation of such pipes is therefore to delay cooling of the oil effluents conveyed, not only during established production for their temperature to be for example at least 40xc2x0 C. on arriving at the surface, for a production temperature at the entrance of the pipe of 70xc2x0 C. to 80xc2x0 C., but also in the case of reduction or even stoppage of production in order to avoid the temperature of the effluents descending for example below 30xc2x0 C. in order to limit the problems mentioned above or at least allow them to be rendered reversible.
Moreover, when such pipes are to be laid at depths greater than 300 meters, the ambient pressure of at least 30 bars prevents the use of high-performance heat insulators which are encountered on land or at shallow immersion, as they all use gases of which the heat conductivity is in effect very low and whose convection is blocked by a solid, porous, cellular or fibrous material: however, the compressivity of the gases does not allow these conventional heat insulators to withstand high outer pressures.
Japanese Patent Application No. JP2176299 published on Oct. 25, 1991 might also be cited, which describes a device for insulating metallic or synthetic resin tubes for supplying hot water in buildings and of which it is desired to conserve the temperature at more than 50xc2x0 C. after one hour of stoppage of supply of hot water, in an ambient temperature of 13xc2x0 C. for example: to that end, it describes a structure comprising a tube for the circulation of hot water, which is preferably deformable to facilitate laying thereof, with a layer of porous material imbibed with paraffin to about 200% and covering it, and another layer of refractory material covering the periphery of the assembly; the use of paraffin makes it possible to have an advantageous coefficient of heat insulation although less than the heat insulators mentioned above and comprising gas, but the capacity of heat accumulation of this Japanese device is reinforced by the presence of the outer refractory layer making it possible to reduce heat loss with the advantage of being able to cut the whole of this structure at any spot in order to facilitate assembly thereof and without loss of the heat accumulation power. However, such a solution cannot be used in water, especially at great depth where it is necessary to be able to withstand a considerable outer hydrostatic pressure, while ensuring sufficient containment in order to avoid any risk of pollution and/or loss of thermal efficiency. Moreover, it does not contribute the specific characteristics described and claimed in the present invention.
Moreover, other specific types of heat insulation compatible with deep immersions have rathermore been developed, which may be grouped in three families, namely:
the outer coatings made of solid plastic such as polyurethane, polyethylene, prolypropylene . . . but whose heat conductivity is fairly average since of the order of 0.2 to 0.3 Watt/meter/degree Celsius, which may be sufficient in continuous production but insufficient to preserve a minimum temperature for a given time in the event of stoppage of production,
the coatings made of syntactic materials constituted by hollow balls containing a gas and resistant to the outside pressure and embedded in various binding agents such as concrete, epoxy, elastomer, polypropylene, etc . . . : the ones with highest performance are the syntactic materials based on epoxy binding agent and on hollow glass microspheres of fairly low conductivity and interesting since of the order of 0.10 to 0.15 watt/meter/degree Celsius, but the cost of these coatings is very high,
the xe2x80x9cpipe in pipesxe2x80x9d in which a first inner tube conveying the effluents is disposed concentrically in a second tube resistant to the outside hydrostatic pressure; the annular space included between the two tubes may either be filled with heat insulator with very low heat conductivity (0.02 Watt/meter/degree Celsius) and which, in order not to be crushed, must be left at atmospheric pressure, or a vacuum may be created therein: such a solution necessitates partitions disposed longitudinally and perfectly tight, at regular intervals, for questions of safety, and complicates the construction and positioning of such assemblies which are, moreover, very expensive.
Another technique consists in prefabricating shells of syntactic foam and in assembling them around the pipe or in making a continuous coating of syntactic foam around said pipe. We would recall on this subject that the syntactic foam is constituted by hollow microspheres containing a gas and bonded by a resin generally of the epoxy type.
These deep-sea insulation technologies use very high-performance products which are extremely expensive and difficult to employ on a large scale.
In the case of installing single pipes or so-called bundles of pipes, it is generally preferred to manufacture said pipes on land in unitary lengths of 500 to 1000 m which are then pulled from the open sea with the aid of a tugboat. In the case of pipes of several kilometers, the first length is pulled, which is joined to the following, the tugboat maintaining the whole in traction during the joining phase, which may last several hours. When all the pipe or bundles of pipes has been put in the water, the whole is towed, generally rubbing on the sea-bed, towards the site, where it is then placed in position.
The insulation of the pipe or pipes or of the bundle is then protected by an outer envelope which has a double function:xe2x80x94on the one hand that of avoiding damage which might occur during towing, which may in certain cases take place over distances of several hundreds of kilometers, which requires using fairly resistant materials such as steel, thermoplastic or thermosetting compound or a composite material;xe2x80x94on the other hand, that of creating a containment around the insulation system.
Such containment is necessary in the case of outer insulating coatings constituted by shells of syntactic foam assembled around the pipes, as the interstices existing between the various shells, as well as the space between the shells and the outer envelope, are filled with a virtually incompressible product, which is generally fresh water or passivated sea water or any other product compatible with the internal components.
In effect, with sea-beds of 2000 m, the hydrostatic pressure is of the order of 200 bars, or 20 Mega Pascals, which implies that all the pipes and the insulating system thereof must be capable of withstanding not only these pressures without degradation during pressurizations and depressurizations of the pipe in which the hot fluid circulates, but also the temperature cycles which will generate variations in volume of the different components as well as of the interstitial fluids, and therefore positive or negative pressures which may lead, if the outer envelope is tight, to partial or total destruction thereof either by exceeding the stresses admissible, or by implosion of this outer envelope (negative variations of internal pressure).
If said outer envelope is not tight, the assembly will then be at equal pressure with respect to the outer pressure, but this will then result in exchanges of fluids between the interior of the bundle and the outer medium. In the case of a filling of the interstices of the bundle with fresh water, passivated sea water or any other product compatible with the internal components as indicated hereinabove, as it is in that case sought to avoid fluid exchanges with the outer medium, one is led to arrange bags constituted by a supple membrane of elastomer type making it possible to contain the variations in volume by maintaining the variations in pressure at a reasonable level but these bags then complicate assembly of the insulating device and do not enable the stresses to be distributed in uniform manner.
The problem raised is therefore that of being able to produce an insulation of at least one underwater pipe intended to be laid on the sea-bed in particular at great depth, of which the insulating coating can withstand not only the hydrostatic pressure but also all the efforts associated with its own weight, and induced upon laying during which the pipe undergoes frictions and is exposed to risks of punching; said insulating coating must make it possible to maintain for example a hot effluent such as an oil product produced for example at 60xc2x0 C. at the level of the sea-bed, at a temperature above for example 40xc2x0 C. when it arrives on the surface after a distance of several kilometers in the water, and, moreover, to maintain a temperature at more than 30xc2x0 C. for example even after several hours of stoppage of production, and this with manufacturing costs which are less than those of present syntactic materials, while offering various possibilities of implementation, and this without risk of pollution for the environment.
One solution to the problem raised is a device for insulating at least one underwater pipe (which may in effect be alone or assembled with other pipes, then constituting what is called xe2x80x9cbundlesxe2x80x9d), intended to be laid on the sea-bed at great depth, comprising an insulating coating surrounding the latter and a protective envelope; according to the invention, said insulating coating comprises a virtually incompressible, liquid-solid phase change material with a melting temperature T0 higher than that T2 of the medium surrounding the pipe in operation and less than that T1 of the effluents circulating in the pipe, which protective envelope is resistant (such as to abrasion, to friction, to corrosion and to mechanical impacts) and deformable (in particular to follow the variations in volume of the phase change material under the effect of the hydrostatic pressure and/or upon variations in temperatures), ensures a containment around said insulating coating, in particular a containment around and against said insulating coating.
Said insulating coating preferably comprises an absorbent matrix surrounding the pipe, preferably nearest the outer surface of said pipe, said matrix being impregnated with said virtually incompressible material.
Said insulating coating may surround the pipe directly or indirectly. In the latter case said insulating coating may surround a pipe itself already insulated, in particular by syntatic foam.
The protective envelope, bearing on the material, solidified and rigid at least on its periphery, is adapted to withstand the weight of the pipe and the frictions when the latter is laid from the surface.
More particularly, the protective envelope is deformable in order to follow the variations in volume of the insulating coating under the effect of the hydrostatic pressure and upon variations in temperature.
This envelope may present at least one gas-permeable vent so as to avoid possible accumulations of gas such as the hydrogen which might diffuse through the wall of the inner pipe, which hydrogen may be generated by the effluents circulating therein.
The object of the present invention is also achieved by a process of insulation using an insulating coating surrounding at least one underwater pipe and a protective envelope, such that:
said pipe is surrounded, preferably directly, with an insulating coating comprising a virtually incompressible, liquid-solid phase change material with a given melting temperature T0, said incompressible material preferably being impregnated in an absorbant matrix, and the whole is contained in the protective envelope which must be resistant and deformable,
there are made to circulate in said pipe hot effluents at a temperature T1 higher than the melting temperature T0 of the material while the ambient outside temperature T2 is less than T0, the phase change material then being liquefied, preferably in a part of the impregnation matrix from the pipe up to a limit of heat exchange equilibrium between the pipe and the envelope, beyond this limit the material being solid,
when the circulation of the effluents in the pipe is stopped, the temperature of these effluents is maintained above a given temperature T3 for a predetermined duration thanks to the heat transfer brought by the latent heat of solidification of the material of which the liquid part solidifies progressively on cooling.
The result is a novel device and process for insulating at least one underwater pipe intended to be laid on the sea-bed in particular at great depth, avoiding the drawbacks mentioned hereinabove in the present known devices and responding to the problem raised. In effect, the outer protective envelope does not need to be resistant to the hydrostatic pressure since it bears on a virtually incompressible material.
Such materials are chosen also to be liquid/solid phase change materials with a melting temperature T0 higher than that T2 of the medium surrounding the pipe in operation in order that its outer part is always solid, and therefore on the one hand resistant enough to absorb the efforts transmitted by the outer protective envelope, on the other hand, contributing a better thermal inertia. Said melting temperature T0 must also be chosen to be less than that of the effluent circulating in the pipe, so that that part of this material surrounding the pipe is liquefied thanks to the calorific addition of the effluents in normal production. On the other hand, in the case of production slowing down or even in the case of stoppage thereof, said phase change material restores its calories to the effluents contained in the pipe, thanks for example to a enthalpy of fusion greater than 50 kilojoule/kilogram, which delays cooling of said effluents by as much and allows the desired objects to be achieved.
Moreover, the outer solid part of said phase change material limits the risks of pollution of this material in the surrounding medium since, in the event of the outer protective envelope tearing, the solidity of this part of the material maintains its integrity.
Said virtually incompressible material is chosen for its low heat conductivity, particularly less than 0.3 watt/meter/degree Celsius.
Said incompressible material is chosen in particular from the materials constituted by at least 90% of chemical compounds chosen from alkanes, in particular comprising a hydrocarbon chain with at least 10 carbon atoms, or salts, hydrated or not, glycols, bitumens, tars, waxes, and other fatty bodies solid at ambient temperature, such as tallow, margarine or fatty alcohols and fatty acids.
The incompressible material is preferably constituted by paraffin comprising a hydrcarbon chain with at least 14 carbon atoms.
The phase change materials described hereinabove generally present a considerable variation in volume during their change of state, which may attain 20% in the case of paraffins. The outer protective envelope must be able to adapt itself without damage to these variations in volume.
Another object of the present invention is therefore also to produce an envelope for protecting the insulation system making it possible, on the one hand, to ensure the integrity of a bundle of pipes and of its insulation system when it is towed and installed in situ and, on the other hand, to overcome the stresses generated by the differential expansion of the various components during the variations in temperature associated with the stresses due to the pressure of the sea bed.
This object is attained by a device for the heat insulation of at least one underwater pipe comprising an insulating coating surrounding the latter and a protective envelope such as according to the present invention, the outer perimeter of the transverse section of said protective envelope is a closed curve of which the ratio of the square of the length over the surface that it defines is at least equal to 13; the protective envelope, which surrounds and ensures containment of the insulating coating, follows the outer shape thereof, and its transverse section which is therefore not circularxe2x80x94as has been usual to do up to the present timexe2x80x94is an oval, a polygon such as a rectangle, or a combination of the two.
In the case of the device comprising at least two pipes disposed in the same plane, the transverse section of said envelope is of shape elongated in the same direction as this plane.
Upon variations in internal volume, the envelope will tend to be deformed towards a circular shape, which mathematically constitutes the shape presenting, with constant perimeter, the largest section.
In the case of a tight envelope of circular profile, an increase in volume generates stresses in the wall, which are associated with the increase in pressure resulting from this increase in volume.
On the contrary, in the case of a rectangular profile, an increase in volume will principally generate flexions of the planar walls, the migration of the interstitial fluids grouping together in these zones of deformation. The efforts generated in the wall will be principally bending efforts in the thickness of the wall, the greatest bendings being produced on the large sides, insofar as the envelope presents a constant thickness over its periphery and over the whole of its length.
By way of example, a circle of diameter 1 m presents an approximate section of 0.785 m2 for an approximate perimeter of 3.1416 m: a square of the same perimeter presenting an approximate section of 0.617 m2, this shape offers a capacity of expansion of 21.5% when, with constant perimeter, the square shape passes to a circular shape; this square shape corresponds to a value of the ratio of the square of the length of its outer perimeter of the closed curve of the transverse section of the protective envelope, with respect to the surface that said perimeter defines, equal to 16.
Similarly, a flattened rectangular shape of which the above ratio is then greater than 16 will present, with constant perimeter, a greater theoretical capacity of expansion; in this way, a rectangle with a large side measuring 1.2 m and a small side of 0.3708 m which has the same perimeter of 3.1416 m, but a section of only 0.44496 m2, presents a theoretical capacity of expansion of 43.3%.
It thus appears that, the more the shape of the transverse section of the insulating coating is flattened, the better the capacity of its envelope is to absorb the expansions due to the expansion of the components under the effect of the temperature.
However, it must not be forgotten that not all of the theoretical capacity of expansion is available, as the deformation by bending of the wall will tend towards the shape of a very imperfect circle. In this way, the effective capacity of expansion is only a portion of said theoretical capacity of expansion and may then correspond to 30% or 50% depending on the quality of the material constituting the envelope, which envelope may be made of steel, thermoplastics or thermosetting compound or of composite material.
Thus, for considerable variations in volumes, profiles will advantageously be used, of which the perimeter of the transverse section of said outer envelope comprises concave reversed curvatures whose concavity is turned towards the outside of said envelope.
In the case of a profile of oval shape, a variation in internal pressure will involve a combination of bending stresses and of pure tensile stresses, as the variable curvature of the oval then behaves as an architectural vault with, however, the difference that, in the case of our envelope, the stresses are tensile stresses and not compression stresses. An oval or approximately oval shape will be thus be able to be envisaged for small capacities of expansion and it will in that case be appropriate to consider ovals with a ratio of length of the large axis xcfx81max over that of the small axis xcfx81min as high as possible, for example at least 2/1 or 3/1.
The shape of the envelope will then be selected as a function of the overall expansion sought of the volume of the insulating coating, under the effect of variations in temperature. In this way, for a system of insulation using principally materials subject to expansion, a rectangular shape, a polygonal shape or an oval shape allow an expansion by bending of the wall while inducing a minimum of tensile stresses in the outer envelope.
For a fluid presenting a great expansion under the effect of variations in temperature, such as gas oil, products of the family of alkanes (paraffins), or phase change materials, the rectangle will advantageously be flattened in order to create the necessary reserve of expansion. This expansion reserve may further be increased by creating the reversed curvatures mentioned hereinabove.
Other advantages of the present invention may be cited, but those mentioned above already show sufficient to prove the novelty and interest thereof. The following description of the Figures concerns embodiments of the invention but has no limiting character: other embodiments are possible within the scope and extent of this invention, in particular by choosing a liquid/solid phase change material specific to the applications desired in terms of depth of immersion, temperature of the ambient medium, temperature of the effluents.